Arduino Mega 2560 + Ramps 1.4 + ATX Power +5v and +12v

Hello gurus.

I am very happy starting my 3D printer project with Arduino.
But now I have one question… And I dont want leard burning my first arduino.

I do not know how to explain what I want do, but I create an image that will demonstrate the system…
I will use LCD and SD module to stand alone operation

My questions:
1 - The Arduino and Ramps will work without USB connected? I will have problems?
2 - What will happen when I connect USB with ATX +5v in same time? Burn all?

Thank you!!!

any one know?